Recently, the demand for stainless steel as a material for coin, medal or the like has increased. The stainless steel for coins is required to have not only excellent corrosion resistance but also good coining workability and wear resistance. That is, the materials for the coin or medal should be soft for facilitating the work during the coining work and should be hard for providing the wear resistance in use after the coining work.
As conventional stainless steel for coins, there are known a strong magnetic ferritic stainless steel as disclosed in JP-A-63-47353, and a non-magnetic austenitic stainless steel as disclosed in JP-A-4-66651.
However, only the strong magnetic ferritic stainless steel has hitherto been used as a material for game coins. Because, when the non-magnetic austenitic stainless steel is used as a material for coins and medals in a game machine, it is difficult to distinguish from currency (money) made of non-magnetic material such as white copper, brass or the like and hence the elimination of forged currency can not be conducted. For instance, if the coin (medal) for the game is used as a currency (money) in a game machine provided with a simple mechanically selecting mechanism instead of an expensive electronically selecting mechanism, discrimination between the coin and the currency is impossible. For this end, the non-magnetic austenitic stainless steel is not used as a material for the game coin.
Another problem with game coins resides in the fact that each of the many game shops desires to have a game coin inherent to each shop. In this case, the size of the coin is actually changed in each shop in order to distinguish the coins made from the same strong magnetic ferritic stainless steel between the shops. However, discrimination through coin size is impractical due to the regulation of the game machines. Furthermore, in order to distinguish the slight difference between coin sizes, an expensive selecting machine having a high selection precision should be used.
In order to solve the above problems, there have recently been proposed gaming coins being adsorbed or not adsorbed through intensity of magnet (maximum energy product BHmax). This coin is made from a weak magnetic material having a middle adequate magnetism between strong magnetism and non-magnetism and enables the discrimination in accordance with the intensity of magnetism. Such a weak magnetic material for the coin is very useful for distinguishing from the currency of non-magnetic material and the gaming coin of strong magnetic material and conducting the discrimination of the coins among many game shops.
As the conventional weak magnetic material, there is used a specific material obtained by incorporating a slight amount of iron powder into brass. However, it is not easy to uniformly disperse the iron powder into the brass. Even if uniform dispersion is attained, directionality is caused in the iron powder by subsequent rolling work and hence there is a problem causing the scattering of the magnetism. Furthermore, the surface of the material is subjected to Ni plating, so that it is expensive, low in work curing and soft, and hence it is easy to cause scratches after the coining.
Heretofore, it has been well-known that meta-stable stainless steels such as JIS-SUS 304 (austenitic stainless steel) and the like produce strain induced martensite through cold work to have magnetism. However, it is common that sufficient magnetism is not obtained only by the coining at a work degree of 15-25%. Furthermore, the stainless steel precipitating the strain induced martensite is relatively high in hardness and has a drawback that the life of the mold is considerably degraded, so that it is not favorable as a stainless steel for coins.
As mentioned above, conventional stainless steels, such as strong magnetic ferritic stainless steel, non-magnetic austenitic stainless steel and martensite precipitated austenitic stainless steel have drawbacks as the stainless steel for coins.
An object of the invention is to propose relatively cheap and weak magnetic stainless steels for coins which are soft and easy to work at raw material stage, are hard after the coining work and have excellent wear resistance and durability and appropriate magnetism for discrimination between coin and currency.
Another object of the invention is to propose a method of advantageously producing coins from the above stainless steels for coins.